Early phases of stellar formation

Processes that promote gravitational collapse and star formation in interstellar clouds are discussed. For example, the increase of the external pressure by passing through the galactic shock, or by compression due to an expanding HII region, supernova shell, etc., may cause a cloud to become gravitationally unstable. Obstacles to star formation are mentioned: thermal pressure, angular momentum, magnetic fields. Processes are proposed to solve the angular momentum problem. In addition to magnetic braking of rotating clouds, magnetic fields are shown to stabilize clouds very effectively. From these properties some speculative conclusions concerning the initial conditions for the presolar nebula and the formation of the cometary cloud are drawn. Numerical examples carried out for self gravitating flows in two and three spatial dimensions are given. They indicate the possibility of converting a cloud's spin into orbital angular momentum by a special fragmentation process. Thus the angular momentum problem is solved by the formation of a binary (or multiple) stellar system. Models including angular momentum transport by turbulent friction and then application to the theoretical evolution of the solar nebula are also briefly mentioned.